Abstract

Male-biased sexual size dimorphism is typical of polygynous mammals, where the degree of dimorphism in bodymass is related to male intrasexual competition and the degree of polygyny. However, the importance of bodymass in monogamous mammals is largely unknown. We investigated the effect of body mass on life-historyparameters and territory size in the red fox (Vulpes vulpes), a socially monogamous canid with slight sexualdimorphism. Increased body size in males appeared to confer an advantage in territory acquisition and defensecontests because heavier males held larger territories and exerted a greater boundary pressure on smallerneighbors. Heavier male foxes invested more effort in searching for extrapair matings by moving over a widerarea and farther from their territories, leading to greater reproductive success. Males that sired cubs outside theirown social group appeared to be heavier than males that only sired cubs within their social group or that werecuckolded, but our results should be treated with caution because sample sizes were small. Territory size,boundary pressure, and paternity success were not related to age of males. In comparison, body mass of femaleswas not related to territory size, probability of breeding, litter size, or cub mass. Only age affected probability ofbreeding in females: younger females reproduced significantly less than did older females, although we did not measure individual nutritional status. Thus, body mass had a significant effect on life-history traits and territory size in a socially monogamous species comparable to that reported in polygynous males, even in the absence oflarge size dimorphism.